If you object to or can't afford paying to visit these great churches, you can always attend evensong at them. They are non-communion services which take place daily at around five in the afternoon (three on Sundays when the choirboys get off early) and last about 45 minutes to an hour. The singing is beautiful and you can see the church in action, lingering afterwards or beforehand to enjoy the architecture.
Westminster Abbey
www.westminster-abbey.org/
20 Dean's Yard London SW1P 3PA
+44(0)20 7222 5152
Google map: bit.ly/VKQrQE
St Paul's:
www.stpauls.co.uk/
St Paul's Churchyard, London, EC4M 8AD
+44(0)20 7236 8350
Google map: bit.ly/Pv4GuK